Brittle composite structure for resiliently accomodating thermal expansion

ABSTRACT

Two composite parts, such as sheet and a hollow gas turbine liner tile, are fastened together using a fastening system that includes a journal bearing that fits in a bore in the sheet and springs to support a bolt fastener that is tightened to hold the parts together. Resilient spacers are placed between the sheet and the liner. The springs may comprise a stack of rings, each ring being a tightly wound helical spring, giving the ring a hollow cross section and resilient qualities.

This invention was made under a United States Government contract andthe United States Government has an interest herein.

This is a division of application Ser. No. 08/360,937 filed on Dec. 21,1994 now U.S. Pat. No. 5,592,814.

TECHNICAL FIELD

This invention relates to techniques for attaching structures, inparticular, brittle composite structures, such as ceramic sheets andexhaust liners in gas turbine engines.

BACKGROUND OF THE INVENTION

Composite parts and structures are finding ever wider use because oftheir high strength to weight ratio. Some composites are attractivebecause they also resist high temperatures. Gas turbine exhaust linersare one environment where these qualities are especially attractive.Ceramic components are especially attractive because they resist theextremely high gas turbine exhaust temperatures, although they do havesignificantly less strength than pure metal components. Ceramiccomposites such as Nextel 440/Silica display a rather brittle matrixstructure, one that tends to powder when exposed to impact and pointloads. That characteristic presents challenges when mounting ceramicmatrix components, such as liners in an engine exhaust. A conventionalbolt can create extremely high point loads, fracturing the ceramicsurface. In other words, the surface can be damaged simply from thecompression if a conventional screw bolt is used to fasten the ceramicin place. In addition, vibration produces mechanical loads that generateimpact and point loads on the ceramic's surface. Similar loads, createdfrom large differences in thermal expansion, can damage the ceramic. Forinstance, thermal radial growth by a bolt in a hole or bore in acomposite structure can exert substantial forces, producing either orboth a crack and bearing failure.

Some of these problems have been considered in other applications in theprior art. For instance, elastomeric bushings are used to reduce noiseand vibration, but they are not resistant to high temperatures and donot provide firm support. Washers are used to distribute compressionloading, but they can distort, creating high point loads. U.S. Pat. No.4,834,569 shows a technique in which an insert or bushing is placed in acomposite core. U.S. Pat. No. 4, 490,083 shows a technique that uses adeformable insert to protect the interface between a fastener and thebore walls. U.S. Pat. No. 4,790,683 shows a method that employs atolerance ring in conjunction with a relatively soft material to preventdamage (deformation) to the material. However, none of these techniquesmake a significant contribution towards protecting brittle ceramiccomposites from all of the forces created by a fastener placed in a hightemperature, high vibration environment, such as if found in the exhaustof a gas turbine engine.

DISCLOSURE OF THE INVENTION

The objects of the present invention include providing a technique forfirmly mounting a composite structure in a way that also optimallyprotects its surfaces from high point loads, stresses from thermalgrowth at the fastener bore and mechanical movement between the fastenerand the structure.

According to the present invention, a fastening arrangement is providedby which a journal bearing is inserted into the bore in a composite. Abearing assembly is inserted into the bore and a fastener extendsthrough the assembly. The assembly includes circular metallic, resilientsprings (bearings with a radially damping effect) that allow axial andradial movement between the fastener and the seat.

According to the invention, the seat contains a flange with acircumferential land so that compression forces are applied to thecomposite at a radial distance away from the edge of the bore.

According to the present invention, when two composite structures arejoined, such as a ceramic sheet supporting a ceramic liner tile, opposedpairs of spring washers are compressed by the fastener. The surfaceswhere the washers contact the structures are covered with an abradablematerial (e.g., a cloth in binder).

According to one aspect of the present invention, the bearing assemblyincludes a stack of annular springs, each formed by tightly winding aflat helical spring in a circular pattern These springs support thefastener in the journal bearing.

According to one aspect of the present invention, the bearing assemblyincludes a plurality of resilient, hollow, cylindrical springs thatsupport the fastener in the journal bearing.

According to the present invention, the bearing assembly includes a wavespring extending around the interior of the seat, separating thefastener and the seat.

According to the present invention, the compressive load from a bolt isapplied at a location radially away from a bore (bolt hole) by the useof a washer with a circumferential land. The land contacts the portionof the flange on the journal bearing containing the land that engagesthe structure's surface.

Other objects, benefits and features of the invention will be apparentfrom the following discussion of one or more embodiments.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified view of a typical gas turbine powered (jet)aircraft in which a cooled, ceramic liner tile, shown in FIGS. 3 and 7,is used in the exhaust.

FIG. 2 is a section showing two composite panels that are fastenedtogether according to the present invention.

FIG. 3 is a section showing a composite (e.g. ceramic) sheet attached toa composite liner according to the present invention.

FIG. 4 is an exploded view of FIG. 2.

FIG. 5 a perspective, shows an insert containing a journal bearing andwave spring bearing for a fastener, according to the present invention.

FIG. 6 is a perspective showing two resilient, metal separators that areused to separate two composite panels that are held together accordingto the present invention.

FIG. 7 is a perspective of an exhaust liner tile mounting arrangementthat embodies the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In FIG. 1, the exhaust 10 of an aircraft 12 is lined with a ceramiccomposite liner 14, such as Nextel 440/Silica. For the purposes ofdiscussion, the exhaust should be assumed to comprise of removablepanels 16 that are sometimes fastened to each other, as shown in FIG. 2,or fastened to a hollow (for air cooling) tile 18, as shown in FIG. 3and FIG. 7, to line the exhaust nozzle.

Referring to FIG. 2, two sheets 16, which are also shown in FIG. 4, areheld together by a bolt 20 that extends through a bore 22 in each panel.Edgings 23 actually define the bore. A journal bearing 26 is placed inthe bore 22, and a plurality of hollow, thin-walled, springs 28 (bearingtubes) radially support the bolt 20. These bearing tubes 28 extend thelength of the journal bearing 26. Being thin walled, they provideresilient radial (lateral) support to the bolt, accommodating thermalexpansion and contraction at the bore 22. In addition, this resiliencyallows the bolt to shift slightly in the journal bearing 26 as it isinserted. One of the washers 30 is placed between the bolt cap 31 andflanges 26a on the journal bearing 26. Another one of the washers 30 isalso placed in a similar manner beneath a nut 32. FIG. 5 demonstratesanother embodiment of the invention describe in FIG. 2 and FIG. 4.There, a wave spring 34, made of pliant metal, is inserted between thebolt 20 and the journal bearing 26. The spring 34 may be tack-welded inthe journal bearing 26 at locations 38, holding it in place.

Turning now to FIG. 3, another embodiment of the invention isillustrated. Once again a composite (ceramic) sheet 16 is used, but inFIG. 3, it is attached to a four sided ceramic tile 40, hollow in thecenter to receive cooling air. The tile is used to line an exhaustnozzle 10a at the end of the exhaust 10. The nozzle 10a is not shown indetail. Many jet engines in use have lined nozzles, though not lined asprescribed by this invention. Annular springs 42 separate the bolt 44from a journal bearing 46 that fits inside the bore 47 in the sheet 16.These springs 42, appearing as small, hollow, metal o-rings, areactually made of a tightly wound helical spring that is formed to becircular in plan and cross section (as shown). Such springs are made byFuron, Corp of Los Alimitos, Calif. They provide a similar type ofsupport to that provided by the bearings 28 in FIG. 4; that is, they canbe squeezed side to side (radial movement), which prevents stressbuildup in the bore as the finer tile 40 heats, along with the bolt 44.The springs 42 also accommodate some axial movement by the bolt in thejournal bearing. The journal bearing 46 contains a flange 46a with acircumferential land 46b that presses against the sheet 16 at someradial distance from the bore 47, which prevents stress concentrationnear the edge of the bore. Likewise a washer 49, between the bolt head44a and the journal bearing flange 46a also contains a circumferentialland 49a that engages only the land 46b. As a result, the compressionforce from the bolt is applied at some radial distance away from thebore.

FIG. 6 shows in greater detail the two somewhat concave spacers 50 thatare located between sheet 16 and line tile 40. Due to the tabs 50a,which give the spacers a concave cross-section, they compress as thebolt 44 is tightened. Another washer 52 also has a circumferentialflange 52a to apply the compression load a some radial distance awayfrom the bore 47 as the nut 54 is tightened. To avoid damage to thecomposite's surface by the concave spacers 50 and the washer 52, thesurface is covered with a fiber mat impregnated with a resilientmaterial 56, e.g., Nextel brand fiber room temperature vulcanizing in(RTV). The spacers 50, serve to, accommodate axial (the direction of thebolt axis) movement between the tile 40 and the sheet 16 and somemovement normal to that direction (radially at the bolt 44).

Finally, FIG. 7 shows the tile 40 attached to an aircraft fuselage 60using the technique shown in FIG. 3. The tile 40, hollow for cooling airflow, protects the fuselage 60 and interior area 62 from hot exhaust gasthat flows by the tile surface 40a. One tile has been shown, but itshould be understood that the nozzle 10a, for that matter other parts inthe exhaust 10, are lined with a multitude of these tiles attached toone or more ceramic sheets 16 using the invention.

From the above explanation, it can be seen that the invention avoidsapplying loads to the sharp edges near the bore in the sheet, provideseffective strain relief from thermal expansion in the bore and throughthe springs 28 and 30 and spacers 50 reduces the application of impactloads due to vibrations to the bore area for composite sheets and linertiles, making the invention especially useful, but not exclusively, togas turbine engine exhaust construction. With the benefit of thisexplanation of the invention, one of ordinary skill in the art may beable to make modifications, in whole or in part to a describedembodiment of the invention without departing from the true scope andspirit of the invention.

We claim:
 1. A composite structure having two composite sheets heldtogether by a fastener inserted through an aligned straight through borein each sheet, characterized by:a bolt having a shank portion and ahead; a journal bearing for insertion into the bore and extending therethrough for receiving the bolt; a plurality of circumferentiallydisposed hollow, resilient springs supporting the bolt mounted betweensaid journal and said shank portion and extending axially and parallelbetween the ends of the journal bearing; and a nut on the bolt remotefrom said head for compressing the sheets between said nut and said headagainst the journal bearing.
 2. A composite structure having twocomposite sheets held together by a fastener inserted through an alignedstraight through bore in each sheet, characterized by:a bolt having ashank portion and a head; a journal bearing for insertion into the boreof one sheet and extending there through for receiving the bolt; aplurality of, resilient annular springs in a stack that extend betweenthe ends of the journal, the bolt passing through the center of saidstack and extending through said aligned bores, said stack supportingthe bolt in the journal bearing, a nut on the bolt for compressing thesheets between said head and said nut against the journal bearing. 3.The composite structure described in claim 2, further characterized inthat:each of said springs comprises a helical spring that is tightlywound to form an annular ring with a hollow cylindrical cross-section.4. The composite structure described in claim 3, further characterizedin that:a pair of opposed resilient spacers separate the two sheets, thebolt extending through each spacer.
 5. The composite structure describedin claim 4, further characterized in that:said journal includes a flangewith a first circumferential land that engages said one sheet and awasher, engaged by a head on the bolt, having a second circumferentialland that engages said flange at a location opposite said firstcircumferential land.
 6. The composite structure described in claim 5,further characterized in that:said spacers rest on a surface on thesheet, said surface comprising a cloth mat impregnated with an adhesive.7. A composite structure having two composite sheets held together by afastener inserted through a bore in each sheet, characterized by:a bolthaving a shank portion and a head; a journal bearing for insertion intothe bore and for receiving the bolt; a wave spring the height of thejournal bearing and attached to the journal bearing for supporting thebolt; and a nut on the bolt for compressing the sheets between said headand said nut against the journal bearing.
 8. A composite structure havetwo composite sheets held together by a fastener inserted through a borein each set, characterized by:a bolt having a shank portion and a head;a journal bearing inserted into the bore for receiving the bolt; meansmounted between said shank portion and said journal bearing, constructedof metal, for resiliently transversely supporting the bolt transverselyin the journal bearing; and a nut on the bolt for compressing the sheetsbetween said nut and said head against the journal bearing.